If I try to write a new value to P0, for example, the instruction is apparently ignored.

1) with a ROM chip, any emulator uses external memory. A good emulaor will map that to behave as internal. THUS, since Philips/NXp makes chips where the regular chip (a bondout is not needed) can be used in emulators (e.g. Ceibo) If you emulator uses one such, the signal applied to !EA is vital GET IT CHANGED.

2) if you took the time to get familar with what you are working with before jumping in you would read "the bible" and know that P0 does not have pullups and thus, without external pullup always will read zero.

3) The microcontroller is a Philips 80C52
Why are you WASTING time and port pins on using a chip with no interanl code memory?

Erik

here are the links to "the bible"
Chapter 1 - 80C51 Family Architecture:
http://www.nxp.com/acrobat_d...ARCH_1.pdf

Chapter 2 - 80C51 Family Programmer’s Guide and Instruction Set:
http://www.nxp.com/acrobat_d...UIDE_1.pdf

Chapter 3 - 80C51 Family Hardware Description:
http://www.nxp.com/acrobat_d...WARE_1.pdf